TEANECK, New Jersey-Respiratory
gating during positron emission
tomography (PET) and computed
tomography (CT) can reduce
uncertainties about tumor location,
thereby improving radiation treatment
planning for patients with lung
cancer, according to a pilot study presented at the 46th Annual Meeting of
the American Society for Therapeutic
Radiology and Oncology (abstract
1056).
"We have been using PET/CT at
our facility for about 3 years now, so
we are very comfortable using functional
imaging with PET in treatment
planning for radiation oncology," said
lead author Allan J. Caggiano, MS, a medical
physicist at Holy Name Hospital,
Teaneck, New Jersey. "We decided
to try to extend this technology
to include imaging the patients while
they are breathing, trying to eliminate
some of the errors associated with
moving tumors under respiration," he
told Oncology News International in
an interview.
Two-Day Protocol
Eighteen patients with lung cancer
have participated in the 2-day protocol.
On the first day, an immobilization
device was made, patients were
trained in regular breathing, and
baseline measures were obtained; on
the second day, ungated and gated
(4D) PET/CT images were obtained,
the latter during coached breathing.
Treatment plans were generated
from both ungated and gated PET/
CTs. Tumor motion on gated PET/
CTs was assessed during 10 phases of
respiration, ranging from one full inhalation
to the next, and was classified
as low (< 1.0 cm), medium (1.0 to 2.0 cm), or high (> 2.0 cm).
Overall, 15 (83%) of the 18 patients
completed the rigorous imaging
protocol. The gated PET and CT
images were successfully fused for
each of the 10 respiratory phases, and
the phases were then combined into a
movie loop.
Analyzing Tumor Motion
Analysis showed that tumor centroids
moved 0 to 3.0 cm during
breathing, and one-third of patients
each had tumors with high, medium,
and low motion. This information is
useful because patients who fall into
the low-motion group probably do
not need any kind of gated treatment,
Mr. Caggiano noted. Not surprisingly,
most of the tumor motion occurred
in the superior-inferior direction.
Clinicians reported that having the
gated PET/CT movie loops helped in
planning radiation therapy and identifying
patients who were good candidates
for respiratory-gated radiation
therapy.
"We found that at least about 50%
of the patients benefited from our
knowing ahead of time what the
movement was so we could establish
that in our treatment margins," Mr.
Caggiano said. "This is a very different
approach from what we have done
in the past, which was to look at how
we thought the tumor was moving in
general for people and then just [expand]
the margins uniformly in all
directions, probably overirradiating
normal lung tissue."
The tailored margins may, in turn,
reduce the adverse effects of radiation
therapy, according to Mr.
Caggiano. Acknowledging the small
study size, he noted that "patients who
have gone through this protocol seem
to have much less respiratory compromise
going out from radiation
therapy onward-a good indicator
that this is probably a good technique
to use for them."
A comparison of the treatment
plans generated from the ungated and
gated PET/CTs revealed that gating
also helped to ensure that all of the
tumor was targeted, especially edges
lying in the main direction of motion,
Mr. Caggiano commented.
"Even with good standard margins on
the ungated field, there are cases when
the tumor moves a lot and traverses
in and out of the path of the ray...So
you would be missing superior and
inferior segments of the tumor and
the patient would get less dose," he
explained.
According to Mr. Caggiano, the
investigators plan to continue developing
their 4D PET/CT process. One
area that they will be looking at in
particular is the consistency of tumor
motion from day to day during treatment
for lung cancer. "PET/CT is a
field ripe for research right now he
concluded. There are many adjustments
that could be made to make it
better."

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